In the technology of prestressing buildings, prestressing with or without bonding is known. In the case of prestressing with bonding, the prestressing tendon, which can consist of several individual tension elements, running in an encasing tube, is, in tensioned state in the finished building, bonded to the concrete by means of a mortar mass introduced into the encasing tube. In the case of prestressing without bonding, the prestressing tendon, or each individual tension element thereof, is usually covered with a plastic sheath, and thereby not directly bonded to the concrete of the building. With correspondingly constructed anchorages such tension elements can be checked, post-tensioned, and if necessary removed or replaced at any time.
In the case of prestressing without bonding the tension elements of the prestressing tendon can be disposed internally in the building or externally. At least one of the ends of each tension element must be held thereby in a stressing anchorage. The other end of each tension element can also be held in a stressing anchorage or in a dead end anchorage.
The tension elements of the prestressing tendon are placed, as a rule, in an encasing tube, which for the most part can run inside the building or outside. Between the stressing anchorage and the end of the encasing tube turned toward the stressing anchorage there is a so-called transitional zone, in which during installation of the prestressing tendon the longitudinal tolerances between the prestressing tendon and the structural part are to be compensated, while maintaining the required corrosion protection and/or the required amount of electrical insulation between the prestressing tendon and the structural part. In the case of a prestressing tendon whose one end is constructed as a rock anchor or an earth anchor, or in the case of a prestressing tendon whose one end terminates in a dead end anchorage, there is a transitional zone at the stressing anchorage only.
The transitional zone between the stressing anchorage and the end of the encasing tube is usually bridged with a so-called trumpet. The trumpet is usually pushed from the outside through the opening of a bearing plate of the stressing anchorage, a flange being provided on the outer end of the trumpet which comes to rest on a stopping face of the bearing plate. In introducing the trumpet through the opening of the bearing plate, the inner end of the trumpet is either pushed over the encasing tube end or comes to lie in the area of the encasing tube end. In the first case, a sealing means is foreseen to achieve the required sealing of the two overlapping ends between the outer wall of the encasing tube and the inner wall of the trumpet. In the second case, a sleeve is fitted over the inner end of the trumpet and the adjacent end of the encasing tube, with which the two ends are joined together tightly. Making such seals is time consuming, and inspections of their tightness not easy. Encasing tubes out of round or having buckles could be the reason for this.
An embodiment of a corrosion-protected stressing anchorage of the type described, in which the trumpet is made of metal and is connected to the encasing tube by means of a sleeve is disclosed in DE 37 34 954. An embodiment in which the inner end of the trumpet is pushed over the encasing tube end, at least the trumpet being made of metal, is shown in AT 388 211.
Furthermore experience with earth and rock anchors has shown that the prestressing tendon is insufficiently protected against stray electrical currents or leakage current. Such currents, upon entering or respectively escaping from the prestressing tendon, can cause electrochemical reactions, such as, for example, a hydrogen embrittlement in the case of entering current and an anodic metal dissolution in the case of escaping current. Residual moisture in the concrete of the structural part is responsible for the entry or respectively the escape of such currents into or out of the prestressing tendon. With the invention described in previously mentioned document, DE 37 34 954, one attempted to achieve sufficient electrical insulation between the strands of the individual tension elements and the concrete structural part by filling the hollow spaces turned toward the stressing anchorage with grease in the transitional zone. This grease protects the strands of the individual tension elements against corrosion, the strands having been freed from their plastic sheaths at this place.
Another solution for achieving an electrically insulated stressing anchorage is shown in the documents U.S. Pat. No. 4,348,844 and U.S. Pat. No. 4,719,658. Proposed therein is to electrically insulate a tensioning wire placed in a plastic tube, or, respectively a tensioning strand sheathed in plastic, in the area of the stressing anchorage in that the stressing anchorage, especially the bearing plate and the load-bearing parts are enclosed with a sheath of electrically insulating material, which sheath is tightly connected to the said plastic tube or to the sheath of the tensioning strands. The outer area of this sheath is sealed with a cap-like cover.
With these measures corrosion of the tension elements and of the stressing anchorage is avoided to a large extent owing to the complete electrical insulation. Based on these solutions the anchor head is completely surrounded by an insulating cover, therefore also the surface of the anchor head which lies on the structural part to support the tension forces. Because of the great load which is transferred from the support surface through the insulation layer of the insulating sheath to the structural part, the danger arises that the insulating material, especially if it consists of plastic, could be damaged before or during setting in concrete or that through the armature force which has its effect through the insulating material, this material is pressed through, above all when it pushes against reinforcement elements. Through damage of this kind stray electrical current or leakage current can arise again. Then a corrosion process is created in places on the stressing anchorage, which makes for a high degree of danger.
Also foreseen in the last-mentioned embodiment is that the insulating cover which surrounds the stressing anchorage is designed in such a way that in order to compensate longitudinal tolerances it has an area which projects over the plastic tube or plastic sheath, respectively, through which the tensioning wire or strand runs. Producing a flawless sealing of this overlapping area between the insulating cover and the plastic tube of the tensioning wire or, respectively, the plastic sheath of the tensioning strand to achieve good electrical insulation is not completely unproblematical since such sealing can only be checked with difficulty. This applies for example when the plastic tube in which the tensioning wire is located is provided with dents or when the cross-section of the tensioning strand with the plastic sheath is not completely circular.